Philippe Pezard, Francois Dominique de Larouziere
A set of new methods to study the transmissive properties of fractured reservoirs in a dynamic sense is presented. The approach is based on electrical methods and conducted from mm-scale in the laboratory to m-scale in boreholes, then extended up to that of the producing reservoir. At cm-scale, in situ measurements made over a few m3 of rocks and borehole wall images permit an analytical quantification in terms of volumes and orientation of fracturing, discriminating between induced and natural fractures, for example.
Due to the very small porosity of most fractured reservoirs, fluids are being drained mostly by the means of fault zones. Keys to the drainage of such zones are found in its geometry and relative orientation to regional stresses. Borehole elongation in front of fault zones is then either an indicator of locally modified mechanical properties, or that of different effective stresses due to high pore pressure. While pore pressure data may be obtained from the individual testing of each zone when identified before hand, indirect information about potential productivity might initially be derived from temperature data.
Detecting high pore pressure zones is certainly one of the keys of the dynamics of such reservoirs, as a strong drawdown may push the system to "lock up", most fractures closing as valves in the absence of sufficient energy to sustain permeability and production. A small decrease of pore pressure may also induce a re-orientation of the local stress field, modifying local drainage directions. In conclusion, the lifetime of the production may be related to the existence of a stress anisotropy by the way of a substantial pore pressure.
AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France